The present invention is directed in part to overcoming the problem of printing on glossy or semi-glossy coated papers or the like with aqueous inkjet inks. Currently available coated papers of this kind have been engineered over the years to be compatible with conventional, analog printing technologies, such as offset lithography, and may be designated as “offset papers.” The printing inks used in offset printing processes are typically very high solids, and the solvents are typically non-aqueous. As a consequence, the coatings that are currently used to produce glossy and semi-glossy offset printing papers, such as those used for magazines and mail order catalogs, have been intentionally designed to be resistant to the absorption of water. In fact, when these papers are characterized by standard tests as to their porosity and/or permeability, they have been found to be much less permeable than a typical uncoated paper.
In contrast to lithographic inks, inkjet inks are characterized by low viscosity, low solids, and aqueous solvent. When such coated offset papers are printed with inkjet inks that comprise as much as 90-95% water as the carrier solvent, the inks have a tendency to sit on the surface of the coating, rather than penetrate into the coating and/or underlying paper substrate.
Because the inks printed on a water-resistant receiver must dry primarily by evaporation of the water without any significant penetration or absorption of the water into the coating or paper, a number of problems are encountered. One such problem is that the individual ink droplets slowly spread laterally across the surface of the coating, eventually touching and coalescing with adjacent ink droplets. This gives rise to a visual image quality artifact known as “coalescence” or “puddling.” Another problem encountered when inks dry too slowly is that when two different color inks are printed next to each other, such as when black text is highlighted or surrounded by yellow ink, the two colors tend to bleed into one another, resulting in a defect known as “intercolor bleed.” Yet another problem is that when printing at high speed, either in a sheet fed printing process, or in a roll-to-roll printing process, the printed image is not dried sufficiently before the printed image comes in contact with an unprinted surface, and ink is transferred from the printed area to the unprinted surface, resulting in “ink retransfer.”
In contrast to glossy offset papers, some coated papers for offset lithography have matte surfaces that are very porous. While high-solids lithographic inks remain on the surface, the colorant of aqueous inkjet inks on the other hand tends to absorb deeply into the paper, resulting in a substantial loss of optical density and as a consequence, reduced color gamut.
Recently high speed inkjet printing processes, including continuous inkjet printing processes, have been developed that are suitable for high speed, mid-volume printing and have become of interest to the commercial printing industry. As commercial offset papers are manufactured in high volume, it would be preferable to be able to use such offset papers themselves for commercial inkjet printing purposes, to take advantage of economies of scale. For the several reasons discussed above, however, the standard preparation of substrates for offset lithographic printing renders them unsuitable for printing with aqueous inkjet inks. Thus the need arises for inkjet-printable receivers providing the familiar look and feel as well as economical cost of standard lithographic printing-grade offset papers.
The requirements of commercial printing industry include, among others, image quality in terms of high optical density, broad color gamut, sharp detail, and minimal problems with coalescence, smearing, feathering and the like. Operationally, the printing process strives for low environmental impact, low energy consumption, fast drying, and so forth. The resulting print must exhibit durability, resisting abrasion when dry or if wetted.
Simply omitting the water-resistant coating of a glossy lithographic offset paper does not enable high-quality inkjet printing. Uncoated paper does not maintain the ink colorant at the surface, but allows significant penetration of the colorant into the interior of the paper, resulting in a loss of optical density and a low-quality image. Moreover, ink penetrates non-uniformly into the paper due to the heterogeneous nature of the paper, giving rise to mottle, which further degrades the image.
Very high quality photopapers have been developed for desktop consumer inkjet printing systems incorporating relatively high laydown ink-receiving layers that are porous and/or permeable to the ink. However, such coated photopapers are generally not suitable for high-speed commercial inkjet printing applications for a number of reasons. The thick coatings result in a basis weight that is impractically heavy for mailing or other bulk distribution means. Such receivers are not meant for rough handling or folding, which would result in cracking of the coated layers. In general, these coated photopapers are too expensive for high-speed inkjet commercial printing applications, such as magazines, brochures, catalogs, and the like. This is because such coated photopapers require either expensive materials, such as fumed oxides of silica or alumina, to produce a glossy surface or very thick coatings to adequately absorb the relatively heavy ink coverage required to print high quality photographs.
Multivalent metal salts are known to improve the print density and uniformity of images formed on plain papers from inkjet printers. For example, Cousin et al., in U.S. Pat. No. 4,554,181, disclose the combination of a water-soluble salt of a polyvalent metal ion and a cationic polymer at a combined dry coat weight of 0.1 to 15.0 g/m2, for improving the print density of images printed by inkjet printers employing anionic dye-based inks.
Varnell, in U.S. Pat. No. 6,207,258, discloses the use of water-soluble salts of multivalent metal ions combined with a polymeric sizing agent and a carrier agent in a size press to improve the print density and uniformity of images formed on plain papers from inkjet printers employing pigment colorants in the ink set. The actual surface concentrations are not readily apparent from the disclosure of the size-press application method.
Tanaka, et al., in U.S. Pat. No. 7,199,182, disclose an inkjet recording material comprising an impervious substrate coated with at least 20 g/m2 of an aqueous resin composition comprising a water soluble magnesium salt, an aqueous polyurethane, and one or more of a cationic compound (such as a cationic polymer), a nonionic water soluble high molecular weight compound (such as acetoacetylated poly(vinyl alcohol) (PVA acac)), and a water soluble epoxy compound.
Tran et al (US 2011/0059272) describe anti-curl compositions for inkjet receivers incorporating a water-soluble salt of a multivalent metal ion and a cationic polymer typically applied in the size press in combination with an amine oxide.
Sheng (US 2003/0203134) describes the use of a low friction substance to be a component of an inkjet receiving layer in order to modify the coefficient of friction of inkjet media and provide improved sheet feeding performance. The low friction substances are emulsified forms of waxes, simple organic polymers, silicone polymers, and fluoropolymers. The particle size is less than 5 micrometers, preferably less than 1 micrometer. The examples shown have slip aids present at approximately 4-5% of the total dry coat weight.
Ma et al. (U.S. Pat. No. 6,779,884) describes a system for printing durable inkjet images in which a slip component is applied over the previously printed inkjet image.
Wang et al. (US 2011/0091666) describes inkjet printable article in which the ink-receiving layer is comprised of 60-95% by weight clays, kaolin, calcium carbonate, or combinations of these particles.
Wang et al. (US 2012/0034398) and Wang et al. (U.S. Pat. No. 8,092,873) each describe an inkjet receptive coating layers comprising inorganic pigments as the major component by weight and a coefficient of friction reducing agent. The latter are described as having a particle size ranging from 0.1-2.0 micrometers, and are comprised of polyethylene, paraffin, carnauba, polypropylene, or PTFE waxes, or combinations of these.
Sargeant et al (U.S. Pat. No. 5,700,582) describes a polymer matrix coating for use as an inkjet receiver layer that avoids the problem of pigment ink cracking. The polymers chosen are characterized by Tg, an integrity value, and a swellability parameter. In addition, up to 15% by weight of the dry coating may be comprised of water-insoluble pigments. These particles may be comprised of organic materials including polyolefins, polystyrene, PTFE, and other materials.
In contrast to incorporating surface modifying chemistry at the size press during paper manufacturing, or coating of a relatively thick ink receiving layer on a photopaper, further coating treatments may also be applied as coatings on finished paper. Dannhauser et al (US 2011/0279554), e.g., describe an inkjet receiving medium including a substrate and having a topmost layer coated thereon comprising an aqueous soluble salt of a multivalent metal cation and a cross-linked hydrophilic polymer binder, for example acetoacetylated poly vinyl alcohol. Xiang and Botros (copending, commonly assigned U.S. Ser. No. 13/433,412) describes an inkjet receiving medium comprising a substrate and having a topmost layer coated thereon, wherein the topmost layer comprises one or more aqueous soluble salts of multivalent metal cations, a cationic polyelectrolyte comprising amidine moieties, and a second polymer which is distinct from the cationic polyelectrolyte comprising amidine moieties and which is selected from the group consisting of a polyamide-epichlorohydrin, a polyamine solution polymer, and a waterborne or water-dispersible polyurethane. Significant improvements in resistance to wet-rub and dry-rub defects may be achieved by use of such coatings on finished paper, along with good image quality, including high optical density and reduced mottle and coalescence.
Nigam, in U.S. Pat. No. 7,041,338 discloses a process of providing a coated paper by utilizing a coating composition containing one or more nitrogenous dye-fixing compound and a film forming binder, where the coating composition may also include an organic or inorganic cross-linker, and where the coating composition may be applied as a pretreatment (prior to printing), simultaneously with printing, or as an after-treatment.
Pigmented inks have many advantages over dye-based inks when they are printed on traditional paper designed for offset printing. It has been a challenge, however, to produce water resistant images with water-based pigmented inks so there will be no ink smearing when end-users turn the pages of a book with wet fingers or water is in contact with printed matter.
Even with the advances made to date in rub resistance, the need remains for more durable digital prints on offset paper to overcome the effects of handling in production and in use by the consumer.